1. Field of the Invention
The present invention relates to an optical element, an optical element molding die, and an optical element manufacturing method and, more specifically, to an optical element, an optical element molding die, and an optical element manufacturing method, which are suited for manufacturing an optical element through resin molding.
2. Description of the Related Art
Conventionally, an injection molding method using a resin material (plastic and the like) has been employed for manufacturing resin-made lenses.
At the time of performing such injection molding, a molten resin material is injected and filled into a cavity where a lens is molded, through a spool, a runner, and a gate, which are the passageway provided in a die for the resin material.
Then, by cooling the molten resin material filled in the cavity along with the molten material filled in the spool, runner, and the gate, the molten resin material is cured, thereby forming a molded lens item in which the molded part (a lens main body) in the cavity and the molded part in the spool, the runner, and the gate are integrally molded.
However, conventionally, there have been a plurality of minute granular concaves, which are referred to as transcription failure, formed on the surface of the lens.
Such transcription failure of the resin material extremely deteriorates the optical performance of the lens. Thus, there has been a demand for finding an effective solution.
In order to overcome the problem of such transcription failure of the resin material, there has been proposed a manufacturing method as disclosed in Patent Document 1, for example, which aims to form a flat lens face with no transcription failure by adjusting the temperature of the resin material to a preferable value. Patent Document 1: Japanese Patent Unexamined Publication 5-177725.
However, in the invention disclosed in Patent Document 1, there is still a specific limitation in controlling the temperatures of the resin material. Therefore, it is difficult to easily suppress the generation of the transcription failure.
As a result of devoted research, the inventors of the present invention have found that there is a correlation between the ratio of the gate thickness to the lens thickness and the generation rate of the transcription failure, and also a correlation between the ratio of the cross sectional area in the thickness direction of the gate with respect to the cross sectional area of the thickness direction of the lens and the generation rate of the transcription failure, which resulted in inventing the present invention.